Antigen recognition by B cells is mediated by the B cell receptor (BCR), a surface-bound immunoglobulin in complex with signaling components CD79a (Igα) and CD79b (10). Crosslinking of BCR upon engagement of antigen results in phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs) within CD79a and CD79b, initiating a cascade of intracellular signaling events that recruit downstream molecules to the membrane and stimulate calcium mobilization. This leads to the induction of diverse B cell responses (e.g., cell survival, proliferation, antibody production, antigen presentation, differentiation, etc.) which lead to a humoral immune response (DeFranco, A. L., 1997, Curr. Opin. Immunol. 9, 296-308; Pierce, S. K., 2002, Nat. Rev. Immunol. 2, 96-105; Ravetch, J. V. & Lanier, L. L., 2000, Science 290, 84-89). Other components of the BCR coreceptor complex enhance (e.g., CD19, CD21, and CD81) or suppress (e.g., CD22 and CD72) BCR activation signals (Doody, G. M. et al., 1996, Curr. Opin. Immunol. 8, 378-382; L1, D. H. et al., 2006, J. Immunol. 176, 5321-5328). In this way, the immune system maintains multiple BCR regulatory mechanisms to ensure that B cell responses are tightly controlled.
When antibodies are produced to an antigen, the circulating level of immune complexes (e.g., antigen bound to antibody) increases. These immune complexes downregulate antigen-induced B cell activation. It is believed that these immune complexes downregulate antigen-induced B cell activation by coengaging cognate BCR with the low-affinity inhibitory receptor FcγRIIb, the only IgG receptor on B cells (Heyman, B., 2003, Immunol. Lett. 88, 157-161). It is also believed that this negative feedback of antibody production requires interaction of the antibody Fc domain with FcγRIIb since immune complexes containing F(ab′)2 antibody fragments are not inhibitory (Chan, P. L. & Sinclair, N. R., 1973, Immunology 24, 289-301). The intracellular immunoreceptor tyrosine-based inhibitory motif (ITIM) of FcγRIIb is necessary to inhibit BCR-induced intracellular signals (Amigorena, S. et al., 1992, Science 256, 1808-1812; Muta, T., et al., 1994, Nature 368, 70-73). This inhibitory effect occurs through phosphorylation of the FcγRIIb ITIM, which recruits SH2-containing inositol polyphosphate 5-phosphatase (SHIP) to neutralize ITAM-induced intracellular calcium mobilization (Kiener, P. A., et al., 1997, J. Biol. Chem. 272, 3838-3844; Ono, M., et al., 1996, Nature 383, 263-266; Ravetch, J. V. & Lanier, L. L., 2000, Science 290, 84-89). In addition, FcγRIIb-mediated SHIP phosphorylation inhibits the downstream Ras-MAPK proliferation pathway (Tridandapani, S. et al., 1998, Immunol. 35, 1135-1146).
A recently recognized function of FcγRIIb is to serve as a scavenger receptor in the liver, clearing antibody:antigen immune complexes from circulation. FcγRIIb is thus an important component of the classical reticulo-endothelial system. For example, Anderson and colleagues (Ganesan et al., J Immunol 2012) published a study demonstrating that three quarters of mouse FcγRIIb is expressed in the liver, with 90% of it being expressed in Liver Sinusoidal Endothelial Cells (LSEC). Moreover, the authors demonstrated that clearance of radiolabeled small immune complexes (SIC) is significantly impaired in an FcγRIIb knockout strain compared to wild-type mice. This is therefore a natural property of the immune system, which can be accentuated by Fc engineering for enhanced affinity to FcγRIIb.
Of relevance in the present invention are allergic diseases. Allergic diseases and conditions, such as asthma, allergic rhinitis, atopic dermatitis, and food allergy, have become increasingly prevalent over the past few decades and now affect 10-40% of the population in industrialized countries. Allergic diseases profoundly affect the quality of life, and can result in serious complications, including death, as may occur in serious cases of asthma and anaphylaxis. Allergies are prevalent, and are the largest cause of time lost from work and school and their impact on personal lives as well as their direct and indirect costs to the medical systems and economy are enormous. For example, allergic rhinitis (hay fever) affects 22% or more of the population of the USA, whereas allergic asthma is thought to affect at least 20 million residents of the USA. The economic impact of allergic diseases in the United States, including health care costs and lost productivity, has been estimated to amount to $6.4 billion in the early nineties alone.
Most allergic diseases are caused by immunoglobulin E (IgE)-mediated hypersensitivity reactions. IgE is a class of antibody normally present in the serum at minute concentrations. It is produced by IgE-secreting plasma cells that express the antibody on their surface at a certain stage of their maturation. Allergic patients produce elevated levels of IgE with binding specificity for ordinarily innocuous antigens to which they are sensitive. These IgE molecules circulate in the blood and bind to IgE-specific receptors on the surface of basophils in the circulation and mast cells along mucosal linings and underneath the skin. Binding of antigen or allergen to IgE on mast cells, basophils, and other cell types, crosslink the IgE molecules, and aggregate the underlying receptors, thus triggering the cells to release vasoactive and neuronal stimulatory mediators such as histamines, leukotrienes, prostaglandins, brakykinin, and platelet-activating factor. The rapid reaction of the immune system to antigen caused by antibody immune complexes has led to the term immediate or antibody-mediated hypersensitivity reaction, in contrast to delayed or cell-mediated hypersensitivity reactions that are mediated by T cells. IgE-mediated immune reactions are specifically referred to as type I hypersensitivity reactions.
The high affinity receptor for IgE (FcεRI) is a key mediator for immediate allergic manifestations. In addition to mast cells and basophils, the primary mediators of allergic reactions, FcεRI is found on a number of other cell types including eosinophils, platelets and on antigen-presenting cells such as monocytes and dendritic cells. An additional receptor for IgE is FcεRII, also known as CD23 or the low-affinity IgE Fc receptor. FcεRII is expressed broadly on B lymphocytes, macrophages, platelets, and many other cell types such as airway smooth muscle. FcεRII may play a role in the feedback regulation of IgE expression and subsequently FcεRII surface expression.
Since IgE plays a central role in mediating most allergic reactions, devising treatments to control IgE levels in the body and regulating IgE synthesis has been of great interest. Several strategies have been proposed to treat IgE-mediated allergic diseases by downregulating IgE levels. One strategy involves neutralizing the IgE molecules by binding the ε-chain of IgE in or near the Fc-receptor binding site. For example, Omalizumab (Xolair) is a recombinant humanized monoclonal anti-IgE antibody that binds to IgE on the same Fc site as FcεRI. Omalizumab causes a reduction in total serum or circulating IgE in atopic patients, which attenuates the amount of antigen-specific IgE that can bind to and sensitize tissue mast cells and basophils. This, in turn, leads to a decrease in symptoms of allergic diseases. Interestingly, serum IgE levels increase after start of therapy because of omalizumab-IgE complex formation and may remain high up to a year after stopping therapy. Consequently, this issue may lead to false-negatives on diagnostic tests and therefore IgE levels must be routinely checked. Accordingly, there exists a need for improved methods and compositions to reduce IgE-mediated diseases and disease symptoms.
Of additional relevance in the present invention is the fact that antibody/antigen immune complexes are well established mediators of inflammation in various autoimmune diseases. Moreover, circulating immune complexes can be deposited in the kidney, ultimately resulting in nephritis, the leading cause of death in systemic lupus erythematosus (SLE). Finally, nucleic-acid (RNA or DNA) containing immune complexes, observed most notably in SLE, can interact with toll-like receptors (TLRs) on immune cells, inducing the release of inflammatory cytokines such as interferon alpha, contributing to disease pathogenesis. The complement system naturally recognizes these antibody-antigen immune complexes (ICs), resulting in complement-component C3 ‘tagging’ of the immune complexes with a variety of fragments of C3 (including C3b, C3b(i), C3d, and C3g). Under healthy conditions, these tagged immune complexes are cleared through interaction with a variety of complement receptors and FcγRs. C3b-C3b-IgG covalent complexes are immediately formed on interaction of serum C3 with IgG-IC. These C3b-C3b dimers constitute the core for the assembly of C3/C5-convertase on the IC, which are subsequently converted into iC3b-iC3b-IgG by the complement regulators. Further processing of iC3b can occur through interaction with these regulators, to produce C3d and C3g. ICs tagged with various forms of C3 have been detected in a variety of autoimmune disease, and C3d-IC levels in particular have been shown to correlate directly with disease activity level in SLE. See Toong C, Adelstein S, Phan T G (2011) Int J Nephrol Renovasc Dis “Clearing the complexity: immune complexes and their treatment in lupus nephritis,” 4:17-28, which is hereby incorporated by reference in its entirety and in particular all figures, legends and disclosure related to models of DNA-anti-DNA immune complex generation and glomerular damage in lupus nephritis and potential therapeutic targets. See also Sekita K, Doi T, Muso E, Yoshida H, Kanatsu K, Hamashima Y (1984) Clin Exp Immunol “Correlation of C3d fixing circulating immune complexes with disease activity and clinical parameters in patients with systemic lupus erythematosus,” 55(3):487-494, which is hereby incorporated by reference in its entirety and in particular all figures, legends and disclosure related to CIC levels and anti-C3d assays from patients with various diseases.
The natural receptor for C3d is the complement receptor 2 (CR2), also known as CD21, expressed on the surface of B cells. CR2 serves as a link to from the innate to the adaptive immune system, and in healthy conditions, the interaction of C3d-tagged immune complexes leads to an amplified B cell/antibody response to the offending antigen. Unfortunately, in autoimmune diseases this amplification can lead to continuation of an auto-antibody response to autoantigen, further exacerbating the disease.
Soluble CRs and CR-Fc fusions have been described for therapeutic purposes. These include CR1, CR2-Fc (U.S. Pat. No. 6,458,360), CR2-fH (CR2-factor H), and others. However, while these approaches generally block interaction of C3-tagged ICs with their associated receptors, they do not necessarily remove the immune complexes from circulation. Most of the complement receptors and regulatory proteins are composed of one or more so-called short complement repeat (SCR) domains, also called complement control protein (CCP) modules or Sushi domains. Typically, only a subset of the domains is involved in direct recognition of the associated complement fragment ligand. For example, it has been demonstrated that only the first two SCRs of CR2 are essential for C3d binding. The SCR domains are stable and well-behaved, making them suitable for use in the development of therapeutic proteins.
Of further relevance to the present invention relates to the mechanisms of hemophilia. One issue with hemophiliacs is the effect that Factor VIII (FVIII (not to be confused with “Fv”)) inhibitors play in disease. Currently, these FVIII inhibitors (generally FVIII antibodies, as shown in FIG. 28) are a huge problem for hemophiliacs.